The function of Histocompatibility Complex (MHC) glycoproteins is to bind peptides derived from foreign antigens and display them on the cell surface for T-cell recognition. Unlike class I MHC molecules, which are involved in the recognition of cytoplasmic proteins, class II glycoproteins bind and display peptides derived predominantly from exogenous proteins. Experiments during the previous grant period suggest a model in which immature class II alphabeta dimers are prevented from binding peptides by a third glycoprotein, the Invariant (1) chain, which associates with them in the endoplasmic reticulum (ER). The I chain remains associated with class II molecules during intracellular transport until it is proteolytically degraded. This occurs in an endosomal compartment to which internalized antigens are directed, and to which class II alphabetaI complexes are transported following exit from the Golgi apparatus. Peptides derived from the internalized antigens are generated in this compartment and bind to the class II glycoproteins following I chain dissociation. In this proposal the molecular and cellular mechanisms which regulate this process will be investigated. The precise subunit structure of human class II (HLA-DR) complexes will be established. The molecular features of the I chain which a) allow it to prevent binding peptide by class II molecules, b) are responsible for its retention and trimerization in the ER in the absence of class II association, and c) are required for class II association, will be examined. The potential role of the I chain in directing class II molecules to the endosomal compartment will be evaluated. Human B-cell lines, expressing transfected genes encoding a membrane bound IgM specific for the trinitrophenyl (TNP) hapten, will be used to investigate the mechanisms involved in the internalization and degradation of protein antigens. Multivalent synthetic antigens with defined HLA-DR-binding peptides chemically coupled to carrier proteins will be used to follow the fate of an individual epitope from its internalization as part of a macromolecule, through proteolysis and association with class II molecules, to its expression on the cell surface for recognition by specific, HLA-DR-restricted, T-lymphocytes.